Role of exogenous Ca in cellular signal transduction and Ca homeostasis
Apart from acts as a nutrient element and structural component, Ca also plays an intracellular second messenger role in plant, and participates in a diverse of signal transduction pathways and biological processes.9 Our study showed that exogenous Ca markedly increased the expression of an important Ca receptor and Ca signal transducer, calmodulin (CAM), at both gene transcription level and proteomic level (Figure 5 and Table 1). Besides, the gene expression of other three Ca receptors and signal transduction related proteins such as Ca-dependent protein kinase 1 (CDPK1 ), calcineurin B-like protein (CBL ) and touch 3 gene (TCH3 ) were up regulated dramatically by exogenous Ca (Figure 5). The roles of these Ca receptors in Ca mediated various signal transductions, cellular events and biological processes including plant growth and development have been widely studied.9,11,69Adequate exogenous Ca supply ensured sufficient intracellular Ca reservation and Ca ion homeostasis, which is the precondition of various Ca involved signal transduction. On the contrary, Ca deficiency lead to significant down-regulation of these genes (Figure 5), suggesting various Ca signal transduction pathways may be distributed by Ca deficiency, and thereafter result in intracellular Ca ion decline and Ca ion dyshomeostasis.70Consistently, physiological parameter measurement (Figure 1D) showed low Ca treatment decreased P. massoniana leaf Ca content while high Ca treatment markedly increased leaf Ca content.
Calreticulin 3 (CRT3) and calnexin 1 (CNX1) are another two Ca binding proteins, which localized to endoplasmic reticulum (ER) and involved in the transportation and storage of Ca ion within ER.71 Furthermore, the two proteins also function as molecular chaperones in the regulation of protein folding.72Exogenous Ca motivated higher expression of this two genes (Figure 5) implies Ca homeostasis regulation in ER is active under adequate exogenous Ca condition. Moreover, exogenous Ca may accelerate the protein folding and packaging through activating these two molecular chaperones, collaborating with other molecular chaperones such as HSP 70 and PPIase CYP20-2 (Table 1).